The present invention generally relates to magnetic recording and reproducing apparatuses, and more particularly to a magnetic recording and reproducing apparatus which records and reproduces data on and from a control track of a magnetic tape by varying the duty cycle of a control pulse signal.
Recently, there is a proposed method of recording data on a control track of a magnetic tape on a magnetic recording and reproducing apparatus (hereinafter referred to as a video tape recorder or simply VTR) for home use employing the existing standards, by varying the duty cycle of the control pulse signal recorded on the control track without affecting a control operation which is originally carried out in the VTR based on the control pulse signal. The control operation includes a control of a head servo system and/or a control of a tape transport system.
The data which is recorded and reproduced on and from the control track may be comment or character data, address data, index data and the like. For example, the comment or character data are used to display characters in a predetermined area of a reproduced picture, the address data are used to detect an absolute address on the magnetic tape, and the index data are used as a cue for searching a certain position on the magnetic tape.
However, according to the proposed VTR, a switching pulse signal for controlling a control head between a reproducing state and a recording state when recording the data on the control track is formed from positive polarity pulses of the reproduced control pulse signal. For this reason, when a tape speed mode of the VTR is changed from a standard play mode to an extended play mode in which the play time is extended compared to that in the standard play mode, the period of the positive polarity pulses of the reproduced control pulse signal becomes short but the timing of the switching pulse signal remains unchanged from that in the standard play mode. As a result, it is impossible to detect the change in the tape speed mode from the standard play mode to the extended play mode, and there is a problem in that a correct control operation cannot be carried out based on the reproduced control pulse signal because the positive polarity pulses of the extended play mode drop out.
On the other hand, when the tape speed mode of the VTR is changed from the extended play mode to the standard play mode, the timing with which the data on the control track are recorded or rewritten is constant and remains unchanged from that in the extended play mode. For this reason, there is no guarantee that the data on the control track will be correctly recorded by intended data, that is, that the duty cycle of the control pulse signal will be varied to a desired duty cycle when the tape speed mode changes to the standard play mode.